The present invention relates to an optical distance measuring device and a method for optical distance measurement, as used, for example, in 3D cameras.
The technical field of application of the present invention is manifold, as will be described below. Complementary metal oxide semiconductor (CMOS) image sensor technology provides effective options for recording measurement signals in real time at high speed. This is of great importance when capturing three-dimensional (3D) distance images. Pulse runtime methods and methods with continuously modulated light serve here for contactless depth detection. For this, the residual intensity of an infrared laser light reflected by an object is measured. This is also referred to as 3D distance measurement.
Typical applications where optical distance measurement can be used are, for example, three-dimensional inspection/positioning systems, one-dimensional positioning systems, such as high-rack warehouses or filling systems, in the automotive field: systems for automobile interior surveillance, for airbag control, anti-theft systems, lane detection systems, so-called pre-crash sensor technology systems, pedestrian protection or parking assist systems. It is also possible to use optical distance measurement for topographical survey or for the detection of persons or for presence sensor technology. Further fields of application are traffic monitoring/counting, logistics, industrial automation or monitoring of different (danger) areas.
In particular in intelligent airbag release and lane detection, high reliability requirements exist for the distance measurement system. In intelligent airbag control systems, for example, the task of releasing the airbag with delayed intensity has to be solved in dependence on the distance of the passenger. Lane detection has to operate reliably also in fog, darkness, bad weather conditions and extreme situations with oncoming light. This is possible with 3D CMOS image sensors. Since, due to the expected legal pressure, there is or will be a high demand for such intelligent systems on part of the automotive industry, a significant market potential results for this field of application.
The advantageous usage of active lighting in three-dimensional (3D) CMOS cameras for capturing a three-dimensional distance image that can be used, for example, in the automotive field is described in patents DE 19833207 A1, EP 104366 B1 and WO2007/031102 A1.
Existing 3D CMOS image sensors for distance or depth measurement are largely based on the functional principle of an active image point sensor or an active pixel sensor (APS). Here, the temporal opening of an exposure window of the pixel is synchronized with the pulsed release of active scene lighting. With the desired pulse light for active scene lighting, however, a portion of the unwanted background light is also detected. Additionally, the reflectivity of the objects of the scene also influences the portion of the reflected light. Depending on the distance of the object, these factors corrupt the payload signal, partly to a considerable extent. In order to obtain sufficiently exact distance information, several images are captured with the laser (pulse light) turned on or off, as well as with two different effective exposure or shutter times. This approach has several disadvantages. On the one hand, capturing the series image sequences (serial capturing) limits the bandwidth, which is why 3D applications are not possible in high and highest speed applications. Further, by serial reflectance correction, it is necessitated to pulse the laser source twice, which means unnecessary energy doubling, which can collide with requirements regarding eye safety with respect to laser radiation in certain fields of application, such as automobile exterior surveillance.